Method for producing leather fiber slurry



United States Patent Ofifice 3,179,342 Patented Apr. 20, 1965 3,179,342METHOD FGR PRQDUCING LEATHER FTBER SLURRY Harland H. Young, WesternSprings, Edward J. Mailra,

Chicago, and Richard H. Eshbaugh, Hinsdale, 111., assignors to Swift &Company, Chicago, 111., a corpora tion of Illinois No Drawing. Originalapplication Jan. 8, 1957, Ser. No, 632,984, now Patent No. 3,116,269,dated Dec. 31, 1963. Divided and this application July 23, 1962, Ser.

No. 217,775 r 3 Claims. (Cl. 241-4) This application is a division ofapplication Serial No. 632,984 filed January 8, 1957, now Patent No.3,116,200 granted December 31, 1963.

This invention relates to a reconstituted leather product and its methodof manufacture. The product in its preferred form is a sheet ofreconstituted leather fibers which has characteristics closelyapproximating the leather properties of color, odor, flexibility,surface appearance, and warmth of feel or hand.

The idea of reconstituting leather and particularly leather scraps intoa usable sheet form is old and attempts have been made over the years toreduce animal leather to a pulp form and then to reconstitute the fibersinto a sheet resembling a hide. Various approaches have been employed,including processes which employ some of the techniques of the paper andhardboard industries. The procedures utilized in the past fall generallyinto five distinct classes:

Class I.-Leather, with or without other fibrous materials, is finelycomminuted and handled through conventional paper or board makingmachinery without any added binders.

Class II.-Leather processed as in Class I has added to it suchconventional proteinaceous binding materials as glue, casein, blood orhydrolyzed leather, and then is formed under heat and pressure intosheet or other molded form.

Class III.--Leather fibers, alone or admixed with other fibers, areworked into a dough form with various mastics, such as rubber, resin,rosins, oil, grease bitumin, asphalt and the like, followed by moldinginto the desired shape. This technique employs organic solvents or evenwater to make a semi-fluid workable mass.

Class IV.-Leather fibers with or without other fibers, are formed into aporous felt, sometimes with the aid of paper making techniques, whichfelt is then dried prior to saturation with solutions or emulsions ofrubber, resin, waxes, asphalt, and other materials of like nature.

Class V.--Leather fibers, with or without other fibrous extenders, areused as fibrous re-enforcement for plastic sheeting made of any of manyplastic materials, e.g. polyvinyl chloride, polyvinylidene chloride,rubber, polyisobutylene, polyethylene, and other thermoplasticmaterials.

Defects have been present in all of the foregoing leather-containingproducts, to the extent that only those products falling in Class V havebecome commercial realities. The unsized, or unbonded, fabrics or feltsof Class I have a very low resistance to a tearing or rupturing forcebecause of the short fiber lengths and the absence of thefiber-interlocking characteristic of natural leather. The addition of asize or binder as in Class II does not correct these defects and, inaddition, produces a stiff sheet incapable of sharp angle bends. Theproduct of Class II also has poor resistance to tearing and resemblesdried, untanned, and unplasticized hide.

The leather-containing products of Class III above, contain more masticor plasticized binders than they do fibers and since such mastics,except for bitumin and asphalt, are expensive, they have not been widelyused.

The more promising mastic sheets of this type have not used leatherfibers at all but have made use of rags, cotton, wood, or paper pulp.High-grade, heavy-duty, asphalt roofing paper belongs in this group.

The products of Classes 1V and V have been made commercially and haveconsiderable flexibility, high strength and excellent resistance tomoisture and solvents generally. They are little more than fiberreinforced plastic sheetings, however, and in addition to beingexpensive they possess the cold feel and the texture of plasticsheeting. Products of this type have a continuous resin film and henceare impervious and do not breathe as does leather. In the Class Vproduct the leather fibers are used solely for the reinforcement of theplastic material and is used for flooring tiles, luggage and upholstery.However, it does not resemble leather and is not used as a substitutefor leather in luxury items.

We have discovered that with suitable modifications and some additionaltechniques, the practices of the hardboard and paper industries,together with the use of a particular bonding resin, namely, polyvinylacetate, and certain plasticizers, it is possible to manufacture areconstituted leather product with properties and characteristicsunattainable in the many products of the past. The raw material used inthe manufacture of our improved reconstituted leather product is mineraltanned leather, the scraps of which at the present time have littlevalue. These leather scraps or hide pieces, if desired, properlyprocessed may be reconstituted into sheet form of predetermineddimensions and desirable properties. The re constituted leather sheethas many characteristic leather properties relating to color, odor,flexibility, surface appearance, and warmth of feel. The sheet, unlikemost of the products of the prior art, is composed of a maximum amountof leather fibers, with a minimum of binder or resinous adhesive. Thesheet is free from the characteristic resinous varnish-like, tacky, orrubbery surface commonly found in reconstituted leather products using athermoplastic substance as the binding material. The reconstitutedleather features a porosity, or moisture vapor permeability approachingthat of leather, which is unattainable in products where the resin isthe continuous phase of the sheet.

Attempts have been made before to convert tanned leather scrap into anaqueous fibrous pulp or stock and mat that aqueous pulp to form a sheet.The leather fibers have been invariably excessively hydrated, with theresult that the ultimate sheets have proven to be objectionably brittle.Leather placed in water will take up an amount of moisture which may becalled free water. A highly hydrated leather contains not only freewater, but in addition it will carry a relatively large amount ofcombined or bound water. Hydrated leather comprises a hydrophilic,though insoluble, protein, which tends to gelatinize when swollen by thebound water. With the practice of the method of our invention forproducing a fibrous slurry, the fibers in the resulting pulp slurry arenot excessively hydrated even though suspended in an aqueous medium. Aspointed out above, this is highly advantageous since a reconstitutedleather sheet made from excessively hydrated hide fibers is brittle. Inaddition, a pulp made up of highly hydrated fibers has a slow feltingrate, i.e., a longer time is required to drain the water from thefibrous slurry in forming the mat, and in addition it is dimcult,following felting, to express water from a Wet mat made up of hydratedpulp. This is reflected in the tendency of a highly hydrated pulp toflow or spread under pressure rather than to lose free water from themat. In our preferred method for producing a fibrous slurry or stockfrom mineral tanned leather pieces, the leather pieces suspended inwater are passed through a milling zone and within that zone they aresubjected to a 3 brief but intense rubbing or shearing action by'closelyspaced and serrated surfaces. The serrated surfaces effeet a shearingforce upon the leather pieces therebetween, milling the water-carriedleather pieces to a fibrous slurry or pulp. The leather is placed in aslurry or fibrous stock form in a short interval of time, requiring onlya few seconds, generally under ten seconds of the intense milling isadequate. Prolonged milling of the leather is principally. responsiblefor the hydration of the fibers and should be avoided for the bestresults.

We have found that only the mineral tanned leathers, e.g. chrome, alum,zirconium and iron tanned, are suitable for use in the manufacture ofour improved composition. Chrome tanned scrap will normally be usedbecause of its large volume and availability. Iron, alum and zirconiumtanned leathers are not available in amounts sufficient to producecommercial quantities of scrap. There is a profound difference betweenmineral and bark tanned leathers. The former mills readily intowell-defined fibers or fiber bundles, whereas the latter on millingyields an excessively hydrated pulp that conglomerates into slimy,stringy masses that felt very slowly. Reasonable felting time isdependent upon the freeness of the stock and is a commercial necessity.We are of the opinion that a bond is formed between the chromium orother metal ion fixed on the protein fiber and the acetate radicalbonded to the polyvinyl nucleus. This bond partially explains thesuperiority of polyvinyl acetate as a binder for mineral tanned leather.

The lowest moisture content to which the leather has been dried in itsprevious handling is of significance in the preparation of the aqueousfibrous stock. The leather which has an initial or equilibrium moisturecontent within the range of l-25% is most suitable for the preparationof stock having discrete individual fibers or fiber bundles. It has beenour experience that a leather which has say a moisture content at timeof milling of 50% may not be readily disintegrated to provide a stock ofsuitable fineness without the danger of hydrating that leather to thepoint where it will take up an objectionable amount of bound or combinedwater. Actually this critical moisture content is the lowest moisturecontent to which the chrome or other mineral tanned leather has beensubjected, rather than its actual moisture content at the time ofmilling. That is to say, a leather which has been pre-dried to amoisture level within the recommended range of -25% and held there toreach an equilibrium may be placed in water prior to milling. Thisleather will then of course have a moisture content outside of therange, but this is not objectionable providing the leather has not beenheld for a period of time long enough to permit a large amount of thewater to become bound, thus excessively hydrating the leather. Hence itis our recommendation that the leather used in our process have aninitial moisture content within the range of 1025% as such leather maybe readily milled to provide the necessary fine and less hydrated fibersneeded for reconstituted leather sheets having greater strength. Ifdried to a moisture content below 10% there is danger of embrittlementand ultimate production of shorter fibers leading to poor strength inthe final sheet. If not dried to the preferred range, then coarser millsettings are required. Coarser mill settings give coarser fibers andfiber bundles which make weaker leather sheets.

At the outset of our process we prepare a pulp or Water slurry of thefibers described above. The resin, preferably with a plasticizer, isintroduced into the slurry and time allowed to permit precipitation ofthe resin and plasticizer onto the individual fibers and fiber bundles.The slurry is then felted, draining off most of the water, to form afibrous mat. The mat is generally cold pressed to remove more of thewater and the resulting cold pressed felt is then dried to furtheradjust the moisture content. Following drying the felt is hot pressed toactivate the thermol plastic resin. The sheet at this point may, ifdesired, be subjected to further finishing.

It is essentially for the production of a good reconstituted leathersheet that the plasticizer or plasticizers used be thoroughly mixed withthe resin before addition of the resin-plasticizer mixture to theslurry. If the materials arenot thoroughly mixed, the finished productwill exhibit an unsatisfactory finish in having stained or spottysurfaces, and in addition the strength of the product will be impaired.

The aqueous fibrous stock must have a reasonable felting time in orderto provide a commercially acceptable process. This may be had to someextent by raising the temperature of the fibrous stock before felting.We have discovered that the freeness of the stock may be appreciablyincreased through the use ofcertain anionic surfactants added to thestock prior to felting. The preferred class of anionic surfactantcomprises sulfated fatty materials whether derived from vegetable,animal or marine sources.

It is critical that the moisture content of the mat from the feltingoperation be adjusted to a moisture conent within the range of 10-30%before hot pressing to activate the polyvinyl acetate resin. Moisturecontents in excess of 30% give an unsatisfactory bond, and moisturecontents of less than 10% at the time of hot pressing result in arelatively dry board exhibiting an objectionably chalky exterior. In ourpreferred method the wet felt is cold pressed at any suitable pressureremembering that excessively high pressures, rapidly applied, may causeflowing of the felt with resultant fissures or spreading. High pressuresare satisfactory when gradually increased as the water is expresed.Following cold pressing the leather sheet is air dried to completeadjustment of its moisture content to within the recommended range.

It is recommended that the mat, after cold pressing and drying, be hotpressed at pressure within the range of 3001000 psi. at a temperature of300 F. Usually a pressing time of 5 minutes is sufiicient for a moderatepressure of 600 psi. and a temperature of F. With the higher pressuresand temperatures the time of pressing will drop accordingly andsometimes a pressing interval of as little as /2 minute will sufiice. Ifpressures and temperatures at the lower end of the range are employed,the time should be proportionately increased. A reconstituted leathersheet may be manufactured with pressures somewhat in excess of 1000 psi.but very short times of pressing would necessarily be resorted to.Pressures less than 300 p.s.i. could be employed if excessively longperiods of pressure are not objectionable and gen erally, if such lowpressures are used, it is best to increase the amounts of resin andplasticizer incorporated in the sheet.

The reconstituted leather sheet of our invention in its preferred formcomprises chrome leather fibers bonded by plasticized polyvinyl acetate.The total amount of resin and plasticizer is less than the amount ofleather fibers, and preferably the plasticizing composition shouldcontain an aromatic hydrocarbon sulfonamide such as anN-substitutedtoluene sulfonamide. It is characteristic of ourcomposition that the plasticized resin does not constitute thecontinuous phase. An aromatic hydrocarbon sulfonamide, when used inconjunction with a plasticizer incorporated for low temperatureflexibility, will impart a feel very similar to that of natural leatherand will in addition appreciably enhance the bursting and tearingstrength of the sheet. In our preferred reconstituted leather sheet themineral tanned leather fibers make up 60 to 85% of the composition, with10 to 25% of the polyvinyl acetate resin, and 5 to 15% of the of onetable should not, without caution, be compared critically with that ofanother because of variations in testing. The Mullen test has been usedas a general measure of strength and it should be remembered that theresults of the Mullen test vary with temperature, humidity and thicknessof the sheet. Most of the Mullen test data has been run on samplesconditioned at 72 F. and 50% relative humidity. However, several of thetests have been run under other conditions which were constant for theexperiments reported but not necessarily the same conditions thatexisted during other tests. Some differences arise because of thevariation in sheet thickness from test to test. The extent of fiberreduction, the nature of the raw materials, and other variables make itnecessary to generally limit comparison of results within the sameexperimental run. It is no coincidence that the known art and presentcommercial reconstituted leather products feature vegetable tannedleather scrap as the raw material in spite of the fact that chrometanned scrap is much more abundant and available. It is our opinion thatthis is due to the fact that a satisfactory sheet of reconstitutedmineral tanned leather has not been previously prepared.

The properties of the leather itself are responsible for the varioustechniques which we use. The ease with which the leather is reduced tofiber and the type of fiber produced in pulp form is vastly differentdepending upon the tanning method used. Vegetable tanned leather, if drymilled, does not act too differently from chrome tanned stock. In wetmilling, however, the difference is great; vegetable tanned stockproduces a ropy fiber which tends to congeal into long ropy masses whichare highly hydrated, whereas chrome stock produces individual fibersthat tend to remain in free suspension over a wider range of conditions.Vegetable tanned stock will wet more rapidly than does chrome stock andthis is probably due to its greater content of fat liquoring components.Whereas one would expect fats, resins, waxes, and other water repellantsof this type to increase the moisture resistance of leather fiber thereverse is true. Therefore chrome tanned stock characterized usually byan absence of fat liquor or other lubricant does not wet readily andwhen reconstituted into sheet form after pulping will not rehydrateunless soaked in water for a prolonged period of time. When we use athermoplastic resin as a binder for chrome leather fiber we then find agreater ease of hydration which is a function of the amount of resin andthe amount of plasticizer. In other words the combination of resin andplasticizer which we incorporate into our reconstituted sheet functionsnot only as a binder but a fat liquoring agent and lubricant as well.The proper choice of resin and plasticizer is therefore critical in thecontrol of the surface feel and other physical properties of ourreconstituted sheet.

There .are a number of pulp forming machines in addition to theconventional hammer or stone mills which have been used in the past inthe manufacture of aqueous stock in the paper making industry. We havefound that one type of machine is much more suitable than the others inthe preparation of a leather fiber pulp. The type of machine which isparticularly adapted to our needs is a machine which features serratedshearing surfaces with provisions for adjusting the clearance betweenthe two surfaces. In general, then, we require a mill which has serratedplates, one shearing against the other at a predetermined clearance. Weprefer to use the Bauer type mill alone or, in the case where a veryfine pulp is desired, in conjunction with a Jordan mill particularlywhen very thin drapery stock is desired. The larger pieces of the chromeor other mineraltreated leather scrap should be first processed by ahammer mill or shredder or some other rough cutter to reduce the scrapin size so that it can be fed to the Bauer mill with some uniformity offeed. A suitable size of leather piece is that characteristic of theindustrial leather waste known as chrome shavings. There are severaltypes of Bauer mills available and in each type of mill there is arotation of one disc with respect to another. One type of Bauer mill hasa fixed disc against which a like disc rotates. Still another type hasboth discs rotating in opposite directions. The leather pieces suspendedin water or with a stream of water are introtroduced into the millingzone defined by the two closely spaced discs at their axis of rotationand are moved outwardly with the assistance of centrifugal force to theperiphery of the milling zone. During its brief passage through the millthe materal is subjected to an intense shearing or rubbing forcesupplied by the closely spaced revolving serrated discs and by theleather rubbing against itself. This action mills the water carriedleather pieces to an aqueous stock of leather fibers. The dwell time ofthe leather pieces in the milling zone is very brief, an average of twoto ten seconds depending upon the diameter of the discs.

In the milling or grinding step water is used in order to float thestock out of the grinding zone after it has reached the desiredfineness. While the amount of water used can be varied, it must besufiicient at all times so as to force the fibers out of the mill beforesevere heating can take place. It has been our experience thatsatisfactory fiber characteristics are obtained only when the weight ofthe water through the mill with the fiber is at least 4 times andpreferably 10 to 20 times the dry weight of the scrap leather fed.Amounts of water less than 4 times the amount of leather results in an"objectionable heating of the fibers during milling, A single passage ofthe material through the mill will place the leather fibers in asuitable form for subsequent processing. Multiple passages through amill or milling through subsequent or consecutive zones may be resortedto but is not normally needed, and generally extensive milling should beavoided as it will cause an objectionable hydration and/ or breaking ofthe fibers.

Chrome leather scraps vary greatly in their moisture content, from 60%moisture down to as low as 5%, depending upon whether the leather scrapis fresh from the tannery or whether it has been stored a sufficientlylong time so as to dry out. The lowest moisture content to which theleather has been dried in its previous handling is a critical factor indetermining the proper clearance between the shearing surfaces of themill. We know from experience that chrome scrap leather which has beenthoroughly dried, i.e. to less than 15% moisture content, will provide asatisfactory fibrous pulp upon milling in an 8 diameter disc Bauer millwhere the discs are set 0.003"-0.005 apart. Bauer mills are classifiedby the size of their shearing disc and are available with 8", 24", 30and 36" discs. Obviously, the dwell time of the leather in a larger millis somewhat greater than in a smaller mill and accordingly the clearancebetween the discs should be increased for the larger miils. Asatisfactory leather slurry is an aqueous fibrous stock which exhibitssuitable freeness and one which may be felted in reasonable time withcommercial equipment. A leather scrap fresh from the tannery willgenerally contain from 60% moisture and if milled with a setting of0.003"- 0.00'5 in an 8" disc Bauer mill (as the dried leather scrapabove) will produce a pulp that will felt so slowly as to beimpractical. Therefore, stock having such high moisture content must bemilled at a coarser setting, for example, 0.025"0.030". If, however, thehigh moisture stock is predried to a moisture level of 15 then resoakedprior to milling, it behaves as dry stock and may be milled with asetting of 0.003"0.005 in an 8" disc Bauer mill. The disc clearance ineither a Bauer or Jordan mill is a function of the lowest moisture content to which the chrome leather scraps have been subjected rather thanto its moisture content at the time of milling. A reconstituted leathersheet composed of finely milled fibers has a greater strength than asheet TABLE I Minimum moisture content to which leather scrap has beenreduced, per- Optirnum clearances between discs for Bauer Mills ofvarious sizes. Disc diameters and clearances given in niches.

cent

The freeness of a felt increases while strength decreases when the millclearances are increased. The time required to drain the water from thefibrous slurry in the formation of'the leather sheet is known as thefelting time. Time for felting is a matter of choice, but obviously in acommercial operation it may not be unreasonably long and for this reasonsome strength will necessarily be sacrificed to obtain an acceptablefelting time. In some instances slower felts which produce much strongersheets may be desirable for certain uses and if so, felting capacitymust be increased in order to maintain production. The following exampleillustrates the relationship between mill clearances, felting time, andproduct strength.

Example I I Chrome leather shavings, initially having a 50% moisturecontent, were dried in air until the moisture level reached 25%. Thismaterial suspended in water was run through an 8 Bauer mill at theseveral disc settings shown in Table II below and for each millclearance one portion of the fibrous slurry was felted into a controlsheet and the other portion, prior to felting, was treated with apolyvinyl acetate resin-plasticizer mix. Both the control and the resinbonded sheet were cold pressed at 300 p.s.i. to remove free water anddried to a moisture content of 20%. The sheets were hot pressed at 600p.s.i. for two minutes at a temperature of 140 F. The sheets were heldafter hot pressing for a period of time at room temperature tocondition. Table II shows the time required for felting the control andthe the resin bonded sheet for each Bauer mill setting and the Mullentest data on the two completed boards. The time of felting is in secondsand is described as freeness in the table. per square inch. Freeness isthe time in seconds required to pull the free water from a standardvolume of the slurry at a vacuum of Hg.

The Mullen test data is in pounds significant.

. s! increased by a rise in temperature of the slurry. The followingexample illustrates this.

Example II Chrome tanned shavings which had an initial moisture contentof 12% were processed as in Example I using the various clearancesindicated in Table III below between the shearing discs of an 8 Bauermill. Samples of fibrous stocks were heated to temperatures of either F.or 140 F.

Here again it is seen that freeness is poorest and strength is thegreatest at close clearances of the mill discs. As these clearancesincrease, freeness increases and strength decreases. The data of TableIII indicates that heating the fibrous slurry prior to felting increasesfreeness to the point where advantage can be taken of the greaterstrength resulting from the closer mill settings. In the milling ofchrome leather scrap containing 12% moisture, the mill settings for an8" Bauer mill may be in a range of 0.004" to 0.008 if the pulp slurry isheated to 140 F. before felting. If the slurry is not heated then millsettings of 0.010" or greater should be used to obtain a practicalfelting time which will result in an accompanying decrease in strength.

Example III The work of this example illustrated the effect of themoisture content of the mineral'tanned scrap on milling technique.Chrome shavings direct from the tannery and containing 51.1% moisturewere air dried in a tumbling barrel. Samples were withdrawn after 0, 45,90, 120, 150, 160, 180, and 200 minutes of drying time and were found tohave moisture contents respectively of 55.1%, 48.5%, 44.4%, 37.2%,30.6%, 24.8%, 14.6%, and 8% after reaching equilibrium. Each sample waspassed through an 8" Bauer mill at the three disc clearances of 0.005",0.015", and 0.025". The ratio of water to fiber through the mill in eachinstance was approximately 10:1. There was some little difference indwell time among the several samples because the higher moisture stockruns through more rapidly than does lower moisture stock; however, thiswas not considered to be Freeness (time required to form the felt fromthe fibrous stock)- was measured as seconds re- .Resin bonded, Mullen-11-- 424 psi...

From our experience, six minutes felting time is com- .merciallyunacceptable except where an exceptionally high strength felt isdesired. We consider a 2-3 minute. felt to be more practical. As will beseen from an examination of Table II, a -second felt may be had .bymilling 25% moisture level chrome shavings through an 8" Bauer mill witha disc clearance of 0.010".

'for two days.

permitted to dry to equilibrium at room temperatures 7 They were thenhot pressed for 3 minutes at F. and600 p.s.i. The finished sheets wereconditioned at 50% relative humidity and 72 F., and

The freeness of the fibrous slurry in felting may be 7 5 on weighingwere found to contain 16.5% moisture.

The times of felting and results of the Mullen strength not uniformlydispersed throughout the fiber, which will tests are in Table IV. result1n an mferior reconstituted leather sheet. For

TABLE IV Moisture Weight of Freeness content of Clearance Freenessconditioned Mullen inseconds Avg. leather scrap ofdises in seconds feltsat 16.5% test Mullen, p.s.l.

moisture, gm.

0. 005 540 85. 7 144 3. 7 0. 015 390 .86. 79 4a 9 4. 3 0.025 450 90. 8107 4. 2 0. 005 300 78. 128 2. 3 0. 015 210 80.0 81 2. 6 2. 5 0.025 18082.2 70 2.6 0. 005 240 85. 9 101 2. 4 0. 015 180 80. 5 70 2. 6 2. 4 0.025' 105 87. 3 58 2. 1 0. 005 300 80. 4 140 2.1 0.015 150 83.8 85 1.8 1. 8 0. 025 90 83. 5 66 1. 4 0. 005 240 86. 2 118 2. 0 0. 015 80 87. 807 1. 2 1. 5 0.025 t 60 88. 1 53 1.1 0. 005 300 72. 4 190 1. 0 0. 015135 83. 5 97 1. 4 1. 4 0.025 70 83.4 75 .9 0. 005 300 83. 1 183 1. 7 0.015 150 84. 7 112 1. 3 1. 4 0. 025 120 83. 6 05 1. 3 0. 005 510 82. 4180 2. 8 0. 015 210 83. 0 114 1. 8 1. 9 0. 025 90 82. 2 s3 1. 1

Variations in the pH of the fibrous slurry at the time of felting has nosignificant effect on the strength of the board as evidenced by theMullen test, providing excessive acidity or alkalinity is avoided. Thenormal pH of the slurry out of the mill is in the range of 3.5 to 5.0.

It may sometimes be desirableto mix other fibers with the leather pulpand where this is done such other fibers should be first milledaccording to that method which is most applicable. Other fibrous pulpscan be prepared from wood, rags, linters, straw, etc., in the wayscommonly used in the paper pulp industry for these materials. Whenincorporating non-leather fibers as extenders, simple mixing of thefiber slurries should not be resorted to. Mixing is best effected byrunning the combined pulps through a subsequent milling during which thediscs are set relatively wide apart in order to avoid excessive fiberreduction. The addition of nonleather fibers as extenders reduces thetrue leather feel, warmth, flexibility, and appearance to the extentthat such fibers are incorporated in the sheet.

After milling and in the instance where extenders are used, afterthorough mixing of the combined fibers the fibrous stock may be usedimmediately or stored for future use for a reasonable length of time(few. days). If storage space is limited, the milled slurry effluent maybe screened and pressed to remove excess water and stored at aconsistency up to fiber solids. More concentrated slurries do not stirout well and may require remilling when diluted. Excessive millingshould be avoided since it may shorten fiber lengths with correspondingreduction in strength of the final product and increase hydration of thefibers thereby making the end product more brittle.

The fibrous content of the aqueous leather, stock to which theresin-plasticizer mix is added may be varied over a wide range,generally from about 1% to 10%, being limited by the time required forcomplete precipitation of the resin and plasticizer and the diflicultyof mixing heavy pulp slurries uniformly. It has been our experience thata slurry containing, say, only 0.5% leather fibers requires anexcessively long time for complete precipitation and adsorption of theresin-plasticizer emulsion. If there is a milky efiiuent from thefelting operation that is evidence that the expensive resin-plasticizermix has not been Wholly precipitated on the fibers. Pulp slurries of 10%fiber content are difiicult to agitate, and even though the resin willprecipitate rapidly it is best results we recommend slurries containingfrom 1% to 6% dry fiber. Resin precipitation is normally completed in afew minutes but in case of rather dilute slurries continued agitationfor perhaps 15-30 minutes may be required. Normally the leather fibersWill have sufiicient tanning agents, chrome or other mineral tanningmaterial, to effect precipitation of the resin from its emulsion, but inthe event the fibers are insufiicient in tanning agents additionalprecipitants may be added, particularly when working with very dilutefiber slurries. Satisfactory precipitants include the syntans, bark tannins, and the soluble salts of aluminum, iron and chromium. The normalpH of the fibers of pulp, which will be in the range of 3.5 to 5.0, issuitable for these precipi tants. The pH range may be varied withoutimpairment of the strength of the final sheet within the range of about2.0 to 9.0. However, beyond these extremes the tanning agents have atendency to leave the leather with resulting decrease in the freeness ofthe felt stock due to gelatinization.

The free water of the slurry may be separated from the leather fibersthrough the use of any of various available felting machines. Theequipment used in the manufacture of felts for the subsequent processinginto paper, hardboard, pasteboard, fibreboard, insulation board, and thelike, is suitable for our purpose. Batch filters of the Chapman Box typewhich involve flotation of the slurry over a flooded screen bottom,followed by evacuation under vacuum of the water through the screen,leaving a felt thereon, may be used. Continuousfelting machinery such asthe Oliver continuous felter is also adaptable to our processing. TheOliver felter features a rotary drum provided with an outer screensurface and an inner mechanism for evacuation. This drum rotatespartially submerged in the fibrous stock and when the felt reaches thedesired thickness it is removed from the screen as a continuous feltedsheet. The speed of the felting drum may be varied with the feltthickness desired.

The felt is freed of excess water by cold pressing on a screen. Coldpressing may be accomplished by hydraulic pressing with large fiatsurfaces or byrunning the felt through squeeze rolls. The pressureapplied and the rate at which the pressure is applied must be correlatedwith felt freeness to extract water without damage to the felt.Generally speaking, when using large flat surfaces to press the board,pressures in the range of to 500 p.s.i. are satisfactory. Pressuresapplied too quickly may cause fissures or undue spreading or flowing ofthe felt.

.short interval.

of the leather pieces to form a fibrous slurry, greater strength may beimparted to the finished reconstituted leather sheet by more finelymilling the fibers. However, pulp slurries containing fine fibers incontrast to more coarse fibers require longer periods of time in thefelting operation to separate the free water. As mentioned earlier,heating of the slurry prior to felting is known to increase the freenessto some extent. We have found that the freeness of the slurry can beincreased appreciably through use of certain anionic surfactants to thepoint where advantage can be taken of the greater strength resultingfrom closer mill settings of the Bauer mill. The anionic surfactant maybe added to the fibrous slurry before or after the incorporation of theresin-plasticizer mix. The one particular type of anionic surfactantwhich we have found to be satisfactory is sulfated fats, Whether derivedfrom vegetable, animal or marine sources. This group of anionicsurfactants has a marked accelerating effect affording a reduction offrom 50 to 80% in felting time for leather fiber slurries. Cationicsurfactants are unsuitable and will generally decrease freeness withoutappreciably increasing the strength of the finished sheets and can begenerally expected to increase felting times from 30 to 60%. For themost part non-ionic surfactants behave like the cationic surfactants inthat theyincrease. felting time though to a lesser degree.

We prefer to use a continuous drier featuring a flow of Warm aircountercurrent to the movement of the felt.

A drier of this type minimizes case hardening and produces a felt withuniform moisture content at a level suitable for hot pressing. Casehardening of the fibers or felt may result if an excessive temperaturedifference and drying rate should be used. The felt should not be heatedabove 260 F. It has been our experience that a stream of warmair at atemperature within the range of 120 F.

and 250 F. applied to the felt for /2 to 5 hours in a tunnel drier willproduce a felt ready for hot pressing having a moisture content withinthe range of 12 to 25 The felt from the cold pressing operation, if heldat room temperature and at a relative humidity within the range of 20 to90% will in time, usually in excess of 16 hours, reach a moisturecontent within the recommended range of 30%. However, such long periodsof time are not commercially feasible.

Conventional systems of applying heat and pressure are satisfactoryprovided allowances are made for suitable or it may be a series ofpaired rollers arranged as in the paper-making industry. In any event,the press mechanism should be capable of developing a pressure of 300 to1000 psi. for a period of time from /2 to 10 minutes (usually 5 minutesis adequate) with provisions for maintaining the temperature of thepressing surfaces from 120 to 300 F. Temperatures in excess of 300 F.are avoided because spotting or stains due to migration of the polyvinylacetate binder and plasticizer will occur at such I elevatedtemperatures unless the pressure is applied for a If a formula of resinand plasticizer is used which is designed to produce a softer or moreflexible finished felt, it becomes necessary to decrease thetemperature, pressure, and the time under pressure. The variables oftime, temperature, and pressure should be integrated so as to provide apressing cycle which will give maximum strength and most leatherlikeproperties at a minimum cost of equipment, labor, power and otherexpenses.

Obviously, if one Were to use hot rolls or calender rolls typical ofpapermaking machinery, where exposure to temperature is very short orinstantaneous, then temperatures up to 350 F. may be used. Conversely,one could press at F. for hours using very high pres sures, e.g. 3000p.s.i., and get a satisfactory sheet. However, modern production methodswould generally demand the following ranges in order to minimizeproduction costs.

Pressures 300 to 1000 lbs. per square inch.

Temperatures F. to 300 F. Times /2 min. to 10 min. Moisture content 10%to 30%.

The following Example V demonstrates the wide variations in pressure,temperature, and time that may be selected.

Example V Chrome leather shavings with a moisture content of 22% weremilled in an 8" Bauer mill set to a disc clearance of 0.015". Theleather scrap was fed to the mill suspended in 20 lbs. of Water for eachlb. of leather. The pulp was gathered in a drum and found to contain11.5 lbs. of dry fiber. Additional water was added to adjust the fibersolids content to 3%. 3.5 lbs. of polyvinyl acetate emulsion (50%solids) was agitated for 15 minutes while adding 1 lb. of water and aclear solution containing 0.5 lb. of dibutyl phthalate, 0.5 lb. ofN-ethyl toluene sulfonamide, and 0.5 lb. chlorinated biphenyl (60%chlorinated). When the plasticizer and the added water had 'beenincorporated in the polyvinyl acetate emulsion, the

mixture was stirred for an additional 15 minutes and the completedresin-plasticizer mixture was held for 24 hours. The following day theaged resin-plasticizer binder was added slowly to the pulp slurry at 65F. while the slurry was vigorously agitated. Agitation was continued for30 minutes at which time the supernatant liquor cleared, indicatingcomplete adsorption or precipitation of the binder on the leather fiber.Solids content of the final slurry was checked and found to be 3.8%.Separate batches were weighed out and drained to form wet felts. Thefelts were held in a drier supplied with a stream of air at 120-440 F.for 5 hours. The dried felts were held for another 24 hours at roomtemperature and at the end of that period were found to have a moisturecontent of 18.8%. The several felts were then pressed under theconditions of time, temperature and pressure as shown in Table VIfollowing. The Mullen tests were run after the hot pressed felts hadbeen conditioned 16 hours at room conditions of 72 F. and 50% relativehumidity.

' TABLE VI Pressure Temp, Time in Mullen Appearance of felt in lbs/in.F. minutes test 3. 0 340 Satisfactory. 2. 0 355 Do. 1.0 325 Do. 0. 5 300Slightly stained. 220 0.25 310 potted. 240 0.1 270 Do. 140 4. 0 300Satisfactory. 160 3.0 290 Do. 180 2. 0 310 D0. 140 5.0 280 Do. 160 3. 5260 Do. 180 2.0 265 Do.

Spotting or stains due to migration of the binder prevail whentemperatures near 200 F. are used. This is accompanied by some decreasein strength due probably 'to migration of the binder to the surface.This migration is also greatest at higher pressures and longer times. Itcan be seen also that strength increases with pressure and that thepressure and temperature are more critical than time.

The finishing of the grained surface may be achieved by any of thevarious techniques now used in the leather 13 industry. Embossing byheat and pressure may be done before or after applicationof coloring orsurface finishing. The embossing plates may be used in a flat press oras part of a pressure roller system. The moisture content of thereconstituted leather sheet should be in the same optimum range of -30%as recommended for hot pressing. Hence, if the moisture content of thereconstituted leather sheet should be below 10%, it is recommended thatit be rehumidified before imprinting the desired grain.

If a smooth, satin surface instead of an embossed grain is desired, thesheet may be calendered with hot rolls or smooth plate in a hot press inthe fashion used for conventional leather. The preferred temperaturerange for finishing is in the range of ISO-200 F. and the recommendedpressure range is 300-600 p.s.i. The time required for this platingoperation may vary from substantially instantaneous at l80200 F. to 30seconds at 150- 180 F. The moisture content, like that recommended forembossing, is in the range of 10-30% for best results. If excessivetemperatures, pressures or times are employed, the surface of theleather sheet tends to become glazed or resinous due to migration of theplasticized polyvinyl acetate resin to the surface. Migration of theresin to the surface mars the soft, leather-like finish that can be hadwith the use of proper temperature, pressure and time. Typical leatherfinishing steps such as dyeing, pigmentation, lacquering, enamelling,waxing, or polishing can be effected by the methods now in use byleather finishers. However, an advantage afiorded by our method ofmanufacture from pulp slurries is the opportunity to produce intensecolors by dyeing of the fibers in slurry form or dispersion of severalpigments together in the slurry form prior to felting. Interestingmottled or marble effects can be had by mixing different coloredslurries together prior to felting. The several following examplesillustrate some of the various conditions and materials which may beutilized in producing a reconstituted leather sheet in accordance withthe teachings of our process.

Example VI heavy slurry was diluted with water to a consistency of 24gms. of a (50% solid) polyvinyl acetate emulsion were diluted with 4gms. of water. The diluted emulsion was heated to 90 F. and to 'this wasadded a clear solution containing 4 gms. of dibutyl phthalate, 4 gms. ofN-ethyl sulfonamide of mixed orthoand paratoluene, and 4 gms. of a 60%chlorinated biphenyl. The polyvinyl acetate emulsion was continuouslyagitated while the plasticizer solution was poured slowly into it.Mixing was continued for 30 minutes and the resin-plasticizer mix wasthen allowed to stand for 24 hours.

The aged resin-plasticizer mix was added to 8.8 lbs. of the pulp slurry(65? F.). The slurry with the added plasticizer-resin mix was agitatedfor minutes at which time precipitation was completed as evidenced by aclear, non-milky, supernatant liquor.

The slurry was then poured over a screen of a felting box and afterbeing dispersed uniformly was evacuated to form a felt. The Water wasdrained from the mat in approximately 3 minutes. Free water wasexpressed by cold pressing at 300 p.s.i. and the felt dried over nightin air at 80 F. The dry felt, which had a moisture content of 16%, washot pressed for 3 minutes at 140 F. and 600 p.s.i. The following day thefinished sheet was plated by pressing instantaneously at 180 F. and 600'p.s.i. The final reconstituted leather sheet showed an average Mullentest of 470 lbs. per square inch.

14 Example VII This example was handled as in the previous example withthe following variations:

Moisture content of the scrap "percent 15 Disc clearance 0.005" Ratio ofwater to leather 15:1 Consistency of slurry after adjustment "percent 3Weight of slurry used lbs 5.9

Resin emulsion used: 26 gms. polyvinyl acetate (50% solids) Plasticizersused:

1 gm. butyl benzyl phthalate;

7 gms. N-ethyl toluene sulfonamide; and

4 gms. 60% chlorinated biphenyl Resin-plasticizer mixture aging perioddays 2 Time for complete resin precipitation a rnins 30 Temperature offelting slurry F 70 Felting time (freeness) mins. 4 Moisture in driedfelt percent 17 Mullen test p.s.i 520 Example VIII This example washandled as in Example VI With the following variations:

Moisture content of the scrap percent 55 Clearance between discs 0.020"Ratio of water to leather 25:1 Consistency of slurry after adjustmentpercent 1% Weight of slurry used lbs 11.7

Resin emulsions used:

22 gms. polyvinyl acetate (50% solids) and 2 gms. polyvinyl chloridePlasticizers 4 gms. dipropylene glycol dibenzoate;

4 gms. N-ethyl toluene sulfonamide; and

4 gms. O-nitro-biphenyl Resin plasticizer mix was aged for 2 days Fiberslurry and binder stood overnight prior to felting.

Temperature of slurry at time of felting F-.. 150 Felting time(freeness) min 1% Temperature of drying air F-.. Drying time hrs 4Moisture content at time of hot pressing percent 24 Mullen test p.s.i330 Example IX This example was run as Example V1 with the followingvariations;

Alum leather shavings (55%) moisture) were dried, to percent 32Clearance of discs 0.010 Ratio water to leather 20:1 Consistency ofslurry after'adjustment percent 3 /2 Weight of slurry for felting slbs-.. 5.0

Resin emulsion used:

23 gms. polyvinyl acetate (50 solids);

1 gm. polyamide resin Plasticizers:

2 gms. butylbenzyl phthalate;

2 gms. dibutyl phthalate 8 gms. N-ethyl toluene sulfonamide; and

2 gms. 60% chlorinated biphenyl Aging period of resin plasticizer mixdays 2 Precipitation time hr /2 Temperature of slurry at time of feltingF 140 Time of felting (freness) min 1% Temperature of drying air F Timeof drying u hrs.-- 3 /2 Moisture content of felt at time of hot pressingpercent 20 Plating time seconds 5 Mullen test p.s.i 340 Example X Thisexample was run the same as Example VI except for the followingvariations:

Chrome leather shavings (55% moisture) were dried to percent 24Clearance between discs 0.015" Ratio of water to leather 18:1Consistency of pulp slurry after adjustment percent 5 Weight of slurryused lbs 3.5

Resin emulsion used:

22- gms. polyvinyl acetate emulsion (50% solids); 24 gms. polyacrylamidesolution) Plasticizers:

1 gm. diotcyl phthalate; 2 gms. dibutyl phthalate; 5 gms. N-ethyltoluene sulfonamide; and 4 gms. ortho nitro diphenyl Slurry plus binderstood for 1 hour before felting.

Temperature of slurry at time of felting F 100 Time of felting(freeness) rnin 2 /2 Temperature of drying air F 140 Time in drier hrs 3/2 Moisture content of felt at time of hot pressing percent 19Temperature of press F 160 Time in press mins 2 Plating temperature a" F170 Plating time sec Mullen test p.s.i 375 Example XI for the followingvariations:

Chrome leather shavings (55% moisture) were dried to percent 16 Ratio ofwater to leather 16:1 Consistency of slurry after adjustment percent 4.Weight of slurry for felting lbs 4.4

Resin used: gms. polyvinyl acetate emulsion solids) Plasticizers used:

1 gm. diethyl phthalate; 2 gms. dibutyl phthalate; 6 gms. N-ethyltoluene sulfonamide; and V 3 gms. 50% chlorinated biphenyl Precipitationtime min 90 Temperature of slurry at time of felting F 58 Time offelting (freeness) rnin 3 Temperature of drying air F 150 Drying timehours 2 Moisture content of felt at time of hot pressing percent 22Temperature of press F 165 Pressure in press p.s;i 300 Time in press min2 Plating temperature F 175 Mullen test p.s.i 440 Example XII Thisexample was run the same as Example VI except for the followingvariations:

Chrome leather shavings moisture) were dried to percent 28 Clearancebetween discs inch 0.010

Ratio of Water to leather 19:1 Consistency of slurry after adjustmentpercent 4 Weight ofslurry used lbs. 4.4

Plasticizers used:

. 7 /2 gms. ortho nitro biphenyl; and

7 /2 gms. N-ethyl toluene sulfonamide Aging time of resin-plasticizermix days 2 Temperature of slurry at time of felting F Felting time(freeness) min 1% Temperature of drier air F Time in drier hours 3Moisture content of felt at time of hot pressing percent 25 Pressure inpress p.s.i 500 Time in press min 3 /2 Plating temperature F Platingtime -sec 30 Mullen test p.s.i 420 Example XIII This example was run thesame as Example VI except for the following variations:

- Iron leather shavings (55% moisture) were dried to percent 18Clearance between discs inch 0.008 Ratio of Water to leather 20:1Consistency of slurry after adjustment percent 3 Weight of slurry usedlbs 5.9

Plasticizers used:

4 gms. ortho nitro biphenyl;

, 4 gms. N-ethyl toluene sulfonamide; and

4 gms. 60% chlorinated biphenyl Precipitation time hours 1 Temperatureof slurry at time of felting F 60 Felting time (freeness) min 2Temperature of drying air F 140 Drying time hrs 2 /2 Moisture content offelt at time of hot pressing "percent" 21 Temperature of press F 5Pressure in press p.s.i 300 Time in press min 2 Plating temperature FMullen test p.s.i 405 The reconstituted sheet leather of our inventionhas many uses without surface finishing. gasket material or as the innercrown material for metal bottle caps. These uses capitalize upon theresiliency of the material. When finished with suitable coloring orembossing or enamelling or lacquering, all finishes used conventionallyin leather working, various degrees of lustre, water-proofing, scuffresistance, and other properties can be had. The finished product hasthe general appearance of natural leather and is suitable for most anyuse Where resistance to repeated sharp flexing is not required. We donot recommend it for use as shoe uppers or soles as leather used forthese purposes should have high flexibility. .It is suitable, however,for inner soles and sock linings. The material is especially desirableas decorative wall tile or even flooring. It is excellent as a resilientunderlayment for composition flooring material or hardwood floorings.Other uses are for low cost clothing accessories, for example, handbagsand belts, where color variety is desired and extreme wearability is notrequired. In the automotive industry the reconstituted leather sheet maybe incorporated in door and crash panels as well as upholstery for theinside car roof and rear window shelves. This material may be laminatedto a rigid structure of Wood, fiber board or metal in the manufacture ofluggage, providing a leather appearance at low cost. In the furniturefield it is useful as leather inlay for coffee, cocktail and cardtables,

It can be used as 17 breakfront panels, and backs of television cabinetsafter perforation.

In addition to wall tile our product can be made in various thicknesses.Sheets of A thickness are suitable as drapery material or wallpaper,laminates for wallboard or acoustical tile, and inexpensive finishedlaminates for flush doors in harmony with various decorative schemes.

Obviously many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claims.

We claim:

1. An improved method of producing a fibrous slurry characterized byfibers having a relatively low degree of hydration from mineral tannedleather which comprises: passing leather having an initial moisturecontent of 1025% in piece form through a milling zone suspended in waterin a ratio of at least four parts Water to one part of the leather andwithin that zone subjecting the leather pieces to an intense rubbingaction provided by closely spaced and serrated surfaces, said surfaceshaving a rotative motion relative to each other so as to efiect arubbing action upon the leather pieces therebetween, thereby millingsaid water carried leather pieces to an aqueous fibrous slurry, with theleather pieces being placed in a fibrous slurry form within ten secondsof millmg.

2. An improved method of producing a fibrous slurry characterized bycontaining fibers having a relatively low degree of hydration frommineral tanned leather having an initial moisture content in the rangeof 10-25% which comprises: introducing the leather in piece formsuspended in at least six parts of water to one part of leather to thecenter of a milling zone defined by two closely spaced, serratedsurfaces, which surfaces have relatively rotative motion about theircommon axes, moving the leather pieces outwardly of the common axisthrough the milling zone with the assistance of centrifugal force andsimultaneously subjecting said leather pieces to an intense shearingforce provided by the closely spaced surfaces, thereby milling saidaqueous carried leather pieces to a fibrous slurry form by the time theleather reaches the outer periphery of the milling zone.

3. An improved method of producing a fibrous slurry characterized byfibers having a relatively low degree of hydration from mineral tannedleather which comprises: drying mineral tanned leather to less than 15%moisture content; passing said leather in piece form through a millingzone suspended in water in a ratio of at least four parts water to' onepart of the leather and within that zone subjecting the leather piecesto an intense rubbing action provided by serrated surfaces spaced about0.003- 0.005 inch apart, said surfaces having a rotative motion relativeto each other so as to effect a rubbing action upon the leather piecestherebetween, thereby milling said water-carried leather pieces to anaqueous fibrous slurry, with the leather pieces being placed in a fiberslurry form within 10 seconds of milling.

References Cited by the Examiner UNITED STATES PATENTS 1,984,869 12/34Farley et al 241-21 2,03 5,994 3/ 36 Sutherland 24l--2l 2,15 6,320 5/39Sutherland 241-28 1. SPENCER OVERHOLSER, Primary Examiner.

1. AN IMPROVED METHOD OF PRODUCING A FIBROUS SLURRY CHARACTERIZED BYFIBERS HAVING A RELATIVELY LOW DEGREE OF HYDRATION FROM MINERAL TANNEDLEATHER WHICH COMPRISES: PASSING LEATHER HAVING AN INITIAL MOISTURECONTENT OF 10-25% IN PIECE FORM THROUGH A MILLING ZONES SUSPENDED INWATER IN A RATIO OF AT LEAST FOUR PARTS WATER TO ONE PART OF THE LEATHERAND WITHIN THAT ZONE SUBJECTING THE LEATHER PIECES TO AN INTENSE RUBBINGACTION PROVIDED BY CLOSELY SPACED AND SERRATED SURFACES, SAID SURFACESHAVING A ROTATIVE MOTION RELATIVE TO EACH OTHER SO TO EFFECT A RUBBINGACTION UPON THE LEATHER PIECES THEREBETWEEN, THEREBY MILLING SAID WATERCARRIED LEATHER PIECES TO AN AQUEOUS FIBROUS SLURRY, WITH THE LEATHERPIECES BEING PLACED IN A FIBROUS SLURRY FORM WITHIN TEN SECONDS OFMILLING.